A TEM comprises a particle source in the form of an electron source generating a beam of particles in the form of a beam of electrons. The electrons are accelerated to an energy of, for example, between 60 and 300 keV. The acceleration is preferably done by placing the electron source on a high voltage and accelerating them to ground potential. Deflectors and lenses then manipulate the beam so that it irradiates a sample. The sample is typically a thin sample of e.g. a biological material, or a semiconductor material with a thickness sufficiently small that it shows transparency to the electrons. To that end thicknesses between, for example, 30 nm (for samples comprising many high-Z atoms, such as semiconductor material or metallic samples) and 200 nm (for biological samples) are routinely used, although samples as thick as, for example, 1 μm may be used.
The beam, while passing through the sample, interacts with the sample. Some electrons pass through the sample unhindered, some electrons are diffracted, some electrons loose energy and some electrons are absorbed.
The electrons passing through the sample are then imaged on a sensor, such as a fluorescent screen. The fluorescent screen may be part of a camera system using a CCD sensor or a CMOS sensor. However, it is also known to form the image directly on a CMOS or CCD sensor. Such sensors capable of imaging 4000×4000 pixels are commercially available.
The TEM can be used in different ways to obtain information about the sample. In some modes the image is an image of the plane where the sample resides, and the intensity shows a distribution depending on the sample constitution.
In other modes, however, a localized high intensity peak is formed on the sensor independent of sample constitution. This is the case when, for example, a diffraction image or an electron energy loss spectrum (EELS) is formed on the sensor.
When forming a diffraction image on a sensor, a crystalline sample is irradiated with a parallel beam.
Diffraction occurs because the periodic structure of a crystalline solid acts as a diffraction grating, scattering the electrons in a predictable manner. Using the diffraction pattern it may be possible to deduce the structure of the crystal. A crystal with a simple unit cell can be determined using only a small number of spots, while a crystal with a complex unit cell, such as a protein crystal, may need as many as 800 spots to be characterized.
It is noted that a diffraction pattern not only provides information of the crystal and the molecules forming the crystal, and e.g. the position of atoms in the crystal, but it may also provide information of, for example, strain and dislocations in the crystal.
Thus, by imaging not the sample plane, but a plane where the (parallel) beam irradiating the sample is focused, an image is formed in which the position in the image corresponds with the angle under which the electrons leave the sample.
Best information is obtained when the image comprises no, or almost no, electrons that are scattered twice. This so-called “double scattering” is the effect in which scattered electrons are scattered again. To minimize double scattering the amount of scattered electrons should be much smaller than the amount of unscattered electrons. This is achieved by using sufficiently thin specimens, such as films with a thickness of 60-300 nm or less (depending on e.g. the energy of the electrons).
Therefore the spot corresponding with undiffracted electrons, also known as the central spot, shows a much higher intensity than the other spots.
When performing diffraction the image magnification is chosen such that the low intensity peaks have a diameter of approximately 3 pixels, so that the position can be determined with sufficient accuracy.
The intensity of the central peak, expressed in electrons per pixel, can be 103 to 105 higher than the highest intensity occurring in the low intensity peaks.
It is noted that, due to this high dynamic range, the central spot is often saturated, that is: the signal ratio of a weak spot with respect to the central spot is less than the electron flux between the two. In other words: the sensor is not linear for the electron flux in the central spot.
It is noted that the sensor is often allowed to saturate at the central spot, that is: the central spot is allowed to clip.
In EELS an energy spectrum is made of the electrons passing through the sample. Electrons passing through a sample may loose energy by so-named inelastic collisions, e.g. causing an atom from the ground state to an excited state, or by ionizations of an atom through which they pass. In an energy analyser placed between the sample and the camera the beam of particles is passed through an energy dispersive element. A line focus is formed on the camera, the position in the line corresponding with the energy loss of the electrons; and the intensity along the line corresponding with the probability of the corresponding energy loss. It is noted that the full width at half maximum (FWHM) often corresponds with the resolution in eV, also known as the dispersion of the spectrometer
It is noted that an EELS spectrum can be made in a TEM equipped with an in-column filter, or with a post-column filter. An example of a commercially available post-column filter is e.g. the “GIF Tridiem” from GATAN Inc., Pleasanton, Calif., USA. This filter can be mounted on many commercially available TEM's.
As is the case in diffraction, most electrons pass through the sample unhindered, or at least without losing energy.
The so-named “zero energy loss peak” of the line spectrum, also named the “zero-loss peak” shows all electrons with an energy loss of e.g. less than 5 eV. This peak typically shows an intensity that is between 102 and 103 more intense than the intensity at other positions of the energy spectrum, thus farther removed from the zero loss peak.
The high local intensity of diffraction images and EELS spectra is often at the same position on the sensor, as it does not dependent of, for example, sample position, but only on the alignment and excitation of particle-optical elements, such as lenses, deflection systems and energy dispersive elements.
A disadvantage of the high local intensity on the sensor is that this may cause damage on the sensor. This damage may be the result of the peak intensity level (that is: the flux), but may also be the result of the high total dose at the high intensity location as a result of the time integrated high intensity level (that is: the total dose). Fluorescent screens can burn in, resulting in a lower response of a part of the screen. CCD chips and CMOS chips may deteriorate, resulting in, for example, higher noise levels or desensitising of pixels, ultimately leading to one or more dead pixels.
There is a need for a method of imaging images with a high intensity peak without damaging the sensor.